655 research outputs found
NGC1333/IRAS4: A multiple star formation laboratory
We present SCUBA observations of the protomultiple system NGC1333/IRAS4 at
450um and 850um. The 850um map shows significant extended emission which is
most probably a remnant of the initial cloud core. At 450um, the component 4A
is seen to have an elongated shape suggestive of a disk. Also we confirm that
in addition to the 4A and 4B system, there exists another component 4C, which
appears to lie out of the plane of the system and of the extended emission.
Deconvolution of the beam reveals a binary companion to IRAS4B. Simple
considerations of binary dynamics suggest that this triple 4A-4BI-4BII system
is unstable and will probably not survive in its current form. Thus IRAS4
provides evidence that systems can evolve from higher to lower multiplicity as
they move towards the main sequence. We construct a map of spectral index from
the two wavelengths, and comment on the implications of this for dust evolution
and temperature differences across the map. There is evidence that in the
region of component 4A the dust has evolved, probably by coagulating into
larger or more complex grains. Furthermore, there is evidence from the spectral
index maps that dust from this object is being entrained in its associated
outflow.Comment: 10 pages, 10 figures. To appear in MNRAS. Uses mn.sty. Also available
at http://www.astro.phys.ethz.ch/papers/smith/smith_p_m.htm
Astrometric signatures of self-gravitating protoplanetary discs
We use high resolution numerical simulations to study whether gravitational
instabilities within circumstellar discs can produce astrometrically detectable
motion of the central star. For discs with masses of M_disc = 0.1 M_star, which
are permanantly stable against fragmentation, we find that the magnitude of the
astrometric signal depends upon the efficiency of disc cooling. Short cooling
times produce prominent filamentary spiral structures in the disc, and lead to
stellar motions that are potentially observable with future high precision
astrometric experiments. For a disc that is marginally unstable within radii of
\~10 au, we estimate astrometric displacements of 10-100 microarcsec on decade
timescales for a star at a distance of 100 pc. The predicted displacement is
suppressed by a factor of several in more stable discs in which the cooling
time exceeds the local dynamical time by an order of magnitude. We find that
the largest contribution comes from material in the outer regions of the disc
and hence, in the most pessimistic scenario, the stellar motions caused by the
disc could confuse astrometric searches for low mass planets orbiting at large
radii. They are, however, unlikely to present any complications in searches for
embedded planets orbiting at small radii, relative to the disc size, or Jupiter
mass planets or greater orbiting at large radii.Comment: 6 pages, 9 figures, accepted for publication in MNRA
The Molecular Line Opacity of MgH in Cool Stellar Atmospheres
A new, complete, theoretical rotational and vibrational line list for the A-X
electronic transition in MgH is presented. The list includes transition
energies and oscillator strengths for all possible allowed transitions and was
computed using the best available theoretical potential energies and dipole
transition moment function with the former adjusted to account for experimental
data. The A-X line list, as well as new line lists for the B'-X and the X-X
(pure rovibrational) transitions, were included in comprehensive stellar
atmosphere models for M, L, and T dwarfs and solar-type stars. The resulting
spectra, when compared to models lacking MgH, show that MgH provides
significant opacity in the visible between 4400 and 5600 Angstrom. Further,
comparison of the spectra obtained with the current line list to spectra
obtained using the line list constructed by Kurucz (1993) show that the Kurucz
list significantly overestimates the opacity due to MgH particularly for the
bands near 5150 and 4800 Angstrom with the discrepancy increasing with
decreasing effective temperature.Comment: 10 pages, 4 figures, 3 table
Kinetic Scale Density Fluctuations in the Solar Wind
We motivate the importance of studying kinetic scale turbulence for
understanding the macroscopic properties of the heliosphere, such as the
heating of the solar wind. We then discuss the technique by which kinetic scale
density fluctuations can be measured using the spacecraft potential, including
a calculation of the timescale for the spacecraft potential to react to the
density changes. Finally, we compare the shape of the density spectrum at ion
scales to theoretical predictions based on a cascade model for kinetic
turbulence. We conclude that the shape of the spectrum, including the ion scale
flattening, can be captured by the sum of passive density fluctuations at large
scales and kinetic Alfven wave turbulence at small scales
NGC 1333/IRAS 4: a multiple star formation laboratory
We present SCUBA observations of the protomultiple system NGC 1333/IRAS 4 at 450 and 850 μm. The 850-μm map shows significant extended emission which is most probably a remnant of the initial cloud core. At 450 μm, the component 4A is seen to have an elongated shape suggestive of a disc. Also we confirm that, in addition to the 4A and 4B system, there exists another component 4C, which appears to lie out of the plane of the system and of the extended emission. Deconvolution of the beam reveals a binary companion to IRAS 4B. Simple considerations of binary dynamics suggest that this triple 4A-4BI-4BII system is unstable and will probably not survive in its current form. Thus IRAS 4 provides evidence that systems can evolve from higher to lower multiplicity as they move towards the main sequence. We construct a map of spectral index from the two wavelengths, and comment on the implications of this for dust evolution and temperature differences across the map. There is evidence that in the region of component 4A the dust has evolved, probably by coagulating into larger or more complex grains. Furthermore, there is evidence from the spectral index maps that dust from this object is being entrained in its associated outflo
Survey of the ULF wave Poynting vector near the Earth's magnetic equatorial plane
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/101878/1/pdfexplain.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/101878/2/jgra50591.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/101878/3/pdfexplain.tx
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